The present invention relates to a device for dosing a liquid or a slurry or for pumping it in a controlled manner.
Swedish Patent Specification No. 373,206 discloses a device for dosing liquids, a device in which a piston in a cylinder is reciprocated by the force of the pressure of the liquid. A separate valve mechanism controls the valves at the ends of the cylinder in such a manner that the pressure of the liquid to be fed moves the piston alternately in different directions and presses the liquid ahead of it to the dosing point. The arrival of the piston at one end of the cylinder is indicated by means of, for example, an inductive switch. At this time the valve mechanism shifts the pressurized feed flow to the other end of the cylinder, and the piston starts off in the opposite direction, simultaneously forcing the liquid ahead of it through the valve mechanism to the feeding point.
In the above device the liquid feed rate depends on the feed pressure of the liquid and on the viscosity of the liquid. When the feed pressure increases or the viscosity decreases, for example owing to a change in the temperature, the liquid flow rate in the pipe system increases and, likewise, the dosing rate increases. In addition, the dosing rate must be adjusted by adjusting the feed pressure, a procedure which requires complicated equipment.
Swedish Patent Specification No. 380,347 discloses a device of corresponding type for dosing liquids. Compared with the previous patent, this device has, additionally, separate clockwise mechanisms in series with the switches which indicate the position of the piston. Thus, when the piston arrives at one end of the cylinder, the clockwise mechanism delays the shift of the state of the valve mechanism by a predetermined length of time. Thus the piston waits at the end of the cylinder for a predetermined length of time before a new movement. The average dosing rate of the device can be adjusted by adjusting the waiting time.
In the device described above, changes in the feed pressure and viscosity of the liquid produce a change in the dosing rate. The device regulates only the waiting time of the piston at the ends of the cylinder. When the feed pressure or the liquid viscosity changes, the velocity of the piston changes, and thus there is a change in the length of one cycle of the device, i.e. the sum of the time consumed for the movement of the piston from one end of the cylinder to the other and the waiting time. This further changes the average feed rate of the device.
Finnish Patent Specification No. 31 300 discloses a device of a corresponding type for dosing liquid, a device in which a signal for the reciprocating movement of the piston is given by means of an external impulse. When the piston reaches the end of the cylinder, the pressure of the liquid shifts the valve mechanism in such a way that the piston moves back to the initial position, in which the piston remains waiting for the new feed impulse.
The device described above does not indicate at all how completely the piston has time to make a full feeding movement during one cycle. In the event that, owing to a decrease in the liquid feed pressure or an increase in the liquid viscosity, the liquid flow is slowed down and the piston does not have the time to complete the movement before a new feed impulse, a change in the average feed rate is produced without the device itself being capable of detecting or indicating it.
The object of the present invention is to provide a completely automatic, precise and rapid-acting device, operating by means of the pressure of the liquid or slurry for dosing or for pumping in a controlled manner a liquid or slurry. The present invention thus relates to a device of the above-mentioned type, having a piston which reciprocates freely in the cylinder; members connected to the cylinder on the opposite sides of the path of the piston movement, for feeding the liquid or slurry, under pressure, into the cylinder in order to move the piston, and for discharging, when the piston returns, the liquid or slurry from the cylinder by means of the pressure of the liquid or slurry directed into the cylinder to the opposite side of the piston; valve members in the liquid or slurry feed and discharge members, for their alternate opening and closing; switches sensing the piston position, fitted at the ends of the cylinder; and a control unit for adjusting the piston stroke frequency and thereby the volume flow of the liquid or slurry and for transmitting the signals given by the switches to the valve members in order to open and close them at different positions of the piston.